The exhaust manifolds and other exhaust system members of automobiles are run through by high temperature exhaust gas which is exhausted from the engines, so the materials which form the exhaust system members are required to exhibit high heat strength, oxidation resistance, thermal fatigue characteristics, and various other characteristics. Ferritic stainless steels which are excellent in heat resistance are used.
The exhaust gas temperature differs depending on the car model, but is usually 800 to 900° C. or so, while the temperature of exhaust manifolds through which high temperature exhaust gas which is exhausted from the engines run becomes 750 to 850° C.
Due to the recent rise in interest in environmental issues, further strengthening of exhaust gas regulations and improvement of fuel efficiency are underway. As a result, the exhaust gas temperatures may rise to close to 1000° C.
The ferritic stainless steels which have been recently used include SUS429 (Nb—Si steel) and SUS444 (Nb—Mo steel). These are based on addition of Nb and further use addition of Si and Mo so as to improve the high temperature strength and oxidation resistance.
Among stainless steels, austenitic stainless steels are excellent in heat resistance and workability. However, austenitic stainless steels are large in coefficient of thermal expansion, so when used for members such as exhaust manifolds which are repeatedly heated and cooled, thermal fatigue fracture easily occurs.
On the other hand, ferritic stainless steels are smaller in coefficient of thermal expansion compared with austenitic stainless steels, so are excellent in thermal fatigue characteristics and scale spallation resistance. Further, they do not contain Ni, so are lower in material costs compared with austenitic stainless steels and are used for general applications.
Ferritic stainless steels are lower in high temperature strength compared with austenitic stainless steels, so techniques for improving the high temperature strength have been developed.
Ferritic stainless steels which are improved in high temperature strength include, for example, SUS430J1 (Nb steel). This uses the solution strengthening or precipitation strengthening by addition of Nb so as to raise the high temperature strength.
Nb steels, however, have the problems of hardening of the finished sheets, drop in elongation, and low r-value, an indicator of deep drawability.
The hardening of the finished sheets is a phenomenon where the presence of solute Nb or precipitated Nb causes hardening to occur at ordinary temperature.
If the elongation falls or the r-value falls, development of a recrystallized texture is suppressed, so the press-formability and shape freedom at the time of shaping exhaust parts become lower.
Further, Nb is high in material cost. If adding a large amount, the production costs rise.
Further, the Mo which is added to SUS444 is also high in alloy cost. The costs of parts remarkably rise.
If excellent high temperature characteristics could be obtained by additive elements other than Nb and Mo, it would become possible to keep down the amounts of addition of Nb and Mo and provide heat resistant ferritic stainless steel sheets which are low in cost and excellent in workability. Therefore, development of heat resistant ferritic stainless steel sheets kept down in amounts of addition of Nb and Mo is being demanded.
To deal with the rise in exhaust gas temperatures, various materials are being developed for exhaust system members.
PLT's 1 to 4 disclose the art of composite addition of Cu—Mo—Nb—Mn—Si.
PLT 1 discloses to improve the high temperature strength and improve the toughness of stainless steels by the addition of Cu and Mo and to improve the scale spallation resistance by the addition of Mn. PLT 1 shows that addition of 0.6% or more of Mn enables reduction of the amount of scale spallation. However, the scale spallation resistance in the case of exceeding 1000° C.×100 hours is not studied.
PLT 2 shows the art of improving the oxidation resistance of Cu steel by adjusting the additive elements in relation to each other and suppressing the formation of the γ-phase at the steel sheet surface. Results of a continuous oxidation test up to 950° C. are shown.
PLT 3 discloses a method of optimizing the contents of Si and Mn of high Cr steel so as to strikingly improve the repeated oxidation characteristics. However, long term oxidation resistance is not studied.
PLT 4 discloses the art of adjusting the amounts of Mo and W of low Cr steel so as to improve the high temperature strength and oxidation resistance.
The inventors disclosed the art, in PLT 5, of using composite addition of Nb—Mo—Cu—Ti—B so as to cause the fine dispersion of Laves phases and ε-Cu phases and obtain excellent high temperature strength at 850° C. PLT 5 discloses that over 0.6% addition of Mn contributes to improvement of scale adhesion and suppression of abnormal oxidation. The art described in PLT 5 is art for making the oxidation resistance and scale spallation resistance equal to SUS444. Results of oxidation tests at 850° C. and 950° C. are shown.
Further, SUS444 has 2% or so of Mo added, so is high in strength, but cannot handle higher temperatures of over 850° C. Therefore, ferritic stainless steels which have a heat resistance of SUS444 or better are being demanded.
Various materials are being developed for exhaust system members to deal with such demands as well.
PLT 6 studies the method of improvement of thermal fatigue characteristics by control of Cu phases with long axes of 0.5 μm or more to 10/25 μm2 or less and Nb compound phases with long axes of 0.5 μm or more to 10/25 μm2 or less.
PLT's 7 and 8 disclose methods of defining the amounts of precipitates for obtaining not only solution strengthening by Nb and Mo, but also solution strengthening by Cu and precipitation strengthening by Cu (ε-Cu phases) so as to obtain a SUS444 or better high temperature strength.
PLT's 9 and 10 disclose the art of adding W in addition to adding Nb, Mo, and Cu.
PLT 9 discloses the relationship between the Laves phases and ε-Cu phases as precipitates and the high temperature strength.
In PLT 10, B is added for further improvement of the workability.
The inventors disclosed, in PLT 11, the art of using the composite addition of Nb—Mo—Cu—Ti—B so as to cause the Laves phases and ε-Cu phases to finely disperse and obtain excellent high temperature strength at 850° C.